FIGS. 1A and 1B depict conventional magnetic elements 10 and 10′. The conventional magnetic element 10 is a spin valve and includes a conventional antiferromagnetic (AFM) layer 12, a conventional pinned layer 14, a conventional nonmagnetic spacer layer 16 and a conventional free layer 18. Other layers (not shown), such as seed or capping layer may also be used. The conventional pinned layer 14 and the conventional free layer 18 are ferromagnetic. Thus, the conventional free layer 18 is depicted as having a changeable magnetization 19. The conventional nonmagnetic spacer layer 16 is conductive. The AFM layer 12 is used to fix, or pin, the magnetization of the pinned layer 14 in a particular direction. The magnetization of the free layer 18 is free to rotate, typically in response to an external magnetic field. The conventional magnetic element 10′ depicted in FIG. 1B is a spin tunneling junction. Portions of the conventional spin tunneling junction 10′ are analogous to the conventional spin valve 10. Thus, the conventional magnetic element 10′ includes an AFM layer 12′, a conventional pinned layer 14′, a conventional insulating barrier layer 16′ and a conventional free layer 18′ having a changeable magnetization 19′. The conventional barrier layer 16′ is thin enough for electrons to tunnel through in a conventional spin tunneling junction 10′.
Depending upon the orientations of the magnetization 19/19′ of the conventional free layer 18/18′ and the conventional pinned layer 14/14′, respectively, the resistance of the conventional magnetic element 10/10′, respectively, changes. When the magnetization 19/19′ of the conventional free layer 18/18′ is parallel with the magnetization of the conventional pinned layer 14/14′, the resistance of the conventional magnetic element 10/10′ is low. When the magnetization 19/19′ of the conventional free layer 18/18′ is antiparallel to the magnetization of the conventional pinned layer 14/14′, the resistance of the conventional magnetic element 10/10′ is high.
To sense the resistance of the conventional magnetic element 10/10′, current is driven through the conventional magnetic element 10/10′. Current can be driven in one of two configurations, current in plane (“CIP”) and current perpendicular to the plane (“CPP”). In the CPP configuration, current is driven perpendicular to the layers of conventional magnetic element 10/10′ (up or down as seen in FIG. 1A or 1B). Typically, in memory applications, such as magnetic random access memory (MRAM) applications, the conventional magnetic elements 10 and 10′ are used in the CPP configuration.
In order to overcome some of the issues associated with magnetic memories having a higher density of memory cells, spin transfer may be utilized to switch the magnetizations 19/19′ of the conventional free layers 10/10′. Spin transfer is described in the context of the conventional magnetic element 10′, but is equally applicable to the conventional magnetic element 10. Current knowledge of spin transfer is described in detail in the following publications: J. C. Slonczewski, “Current-driven Excitation of Magnetic Multilayers,” Journal of Magnetism and Magnetic Materials, vol. 159, p. L1 (1996); L. Berger, “Emission of Spin Waves by a Magnetic Multilayer Traversed by a Current,” Phys. Rev. B, vol. 54, p. 9353 (1996), and F. J. Albert, J. A. Katine and R. A. Buhrman, “Spin-polarized Current Switching of a Co Thin Film Nanomagnet,” Appl. Phys. Lett., vol. 77, No. 23, p. 3809 (2000). Thus, the following description of the spin transfer phenomenon is based upon current knowledge and is not intended to limit the scope of the invention.
When a spin-polarized current traverses a magnetic multilayer such as the spin tunneling junction 10′ in a CPP configuration, a portion of the spin angular momentum of electrons incident on a ferromagnetic layer may be transferred to the ferromagnetic layer. In particular, electrons incident on the conventional free layer 18′ may transfer a portion of their spin angular momentum to the conventional free layer 18′. As a result, a spin-polarized current can switch the magnetization 19′ direction of the conventional free layer 18′ if the current density is sufficiently high (approximately 107–108 A/cm2) and the lateral dimensions of the spin tunneling junction are small (approximately less than two hundred nanometers). In addition, for spin transfer to be able to switch the magnetization 19′ direction of the conventional free layer 18′, the conventional free layer 18′ should be sufficiently thin, for instance, preferably less than approximately ten nanometers for Co. Spin transfer based switching of magnetization dominates over other switching mechanisms and becomes observable when the lateral dimensions of the conventional magnetic element 10/10′ are small, in the range of few hundred nanometers. Consequently, spin transfer is suitable for higher density magnetic memories having smaller magnetic elements 10/10′.
The phenomenon of spin transfer can be used in the CPP configuration as an alternative to or in addition to using an external switching field to switch the direction of magnetization of the conventional free layer 18′ of the conventional spin tunneling junction 10′. For example, the magnetization 19′ of the conventional free layer 18′ can be switched from a direction antiparallel to the magnetization of the conventional pinned layer 14′ to a direction parallel to the magnetization of the conventional pinned layer 14′. Current is driven from the conventional free layer 18′ to the conventional pinned layer 14′ (conduction electrons traveling from the conventional pinned layer 14′ to the conventional free layer 18′). Thus, the majority electrons traveling from the conventional pinned layer 14′ have their spins polarized in the same direction as the magnetization of the conventional pinned layer 14′. These electrons may transfer a sufficient portion of their angular momentum to the conventional free layer 18′ to switch the magnetization 19′ of the conventional free layer 18′ to be parallel to that of the conventional pinned layer 14′. Alternatively, the magnetization of the free layer 18′ can be switched from a direction parallel to the magnetization of the conventional pinned layer 14′ to antiparallel to the magnetization of the conventional pinned layer 14′. When current is driven from the conventional pinned layer 14′ to the conventional free layer 18′ (conduction electrons traveling in the opposite direction), majority electrons have their spins polarized in the direction of magnetization of the conventional free layer 18′. These majority electrons are transmitted by the conventional pinned layer 14′. The minority electrons are reflected from the conventional pinned layer 14′, return to the conventional free layer 18′ and may transfer a sufficient amount of their angular momentum to switch the magnetization 19′ of the free layer 18′ antiparallel to that of the conventional pinned layer 14′.
Although spin transfer functions, one of ordinary skill in the art will readily recognize that it may be relatively difficult to write to the conventional magnetic elements 10 and 10′. In particular, the magnetization may be difficult to switch at a low current, as will be described. When switching the magnetization 19′ of the conventional free layer 18′ to be parallel to the magnetization of the conventional pinned layer 14′, the conventional pinned layer 14′ acts as a source of spin polarized electrons and the conventional free layer 18′ acts as the target. The spin transfer phenomena in this configuration can be described by modifying the Landau Lifshitz Gilbert (LLG) equation of spin dynamics to include a spin-torque term resulting from the spin-polarized current, as given in the publication: J. A. Katine F. J. Albert, R. A. Buhrman, E. B. Myers and D. C. Ralph, “Current-Driven Magnetization Reversal and Spin-Wave Excitations in Co/Cu/Co Pillars,” Physics Review Letters, vol. 84, p3149 (2000). For the magnetic elements 10 and 10′, the film plane is along the x-y plane. The z-direction is directed upwards (perpendicular to film plane) in FIG. 1B. The dynamics of the total magnetic moment, S, of the conventional free layer 18′ is described by:dS/dt=S×{γ[Heff//x−4πM(S·z)z]−αdS/dt−[I g/e |S|]z×S}  (1)The first term in equation (1) describes the steady state precession of spin moment and includes torque from all the fields acting on the magnetization 19′ of the conventional free layer 18′. The field Heff// includes an anisotropy field (Han), an exchange (Hex) and an applied field (Hap) aligned along easy axis of free layer in the film plane. The 4πM term refers to the out-of-plane demagnetization field for the free layer 18′, acting perpendicular to the plane of the free layer, along the z-direction. The demagnetization field results in a shape anisotropy of 2πM2 for the film. The second term is the phenomenological damping effect. Here α is damping coefficient of the ferromagnet. The third term is the spin-torque acting on the moment of the ferromagnet as result of the spin polarized current I, where g is the spin transfer efficiency.
The switching of the direction of the magnetization 19′ of the conventional free layer 18′ occurs when the torque exerted by the current of spin polarized electrons exceeds the damping, which is described by the αdS/dt term in equation (1). When the torque from the current exceeds the damping, the torque causes an outward precession of the magnetic moment of free layer 18′. As a result, at a critical minimum value of the current (Ic), termed the switching current herein, the magnetization direction of the conventional free layer 18′ is switched. Thus, the magnetization 19′ of the conventional free layer 18′ can switch direction due to spin transfer.
The switching current Ic is the minimum current required to switch the direction of the magnetization 19′ of the conventional free layer 18′ using spin transfer. From energy considerations, the dependence of the switching current on Heff and the demagnetization field is given by:Ic∝αM t(Heff//+2πM)  (2)Thus, the switching current is proportional to the saturation magnetization of the conventional free layer 18′, the thickness (t) of the conventional free layer 18′, the damping coefficient (α), the effective demagnetizing field perpendicular to the plane (through the 2πM term) and the effective field in the plane of the conventional free layer 18′, Heff//.
Although conventional magnetic elements can use spin transfer as a switching mechanism, the switching current is high due to the large value of 2πM term. For a number of reasons, a high switching current is undesirable for magnetic memory application. Accordingly, what is needed is a system and method for providing a magnetic memory element that can be more easily switched using spin transfer at a low switching current. The present invention addresses the need for such a magnetic memory element.